A recording device of an ink jet system, that is, an ink jet printer, is a printer of a type which ejects ink droplets for recording from ejection ports of narrow nozzles arrayed on a record heading and impacts the ink droplets on a recording medium such as paper, thus recording characters or images in the form of dots. This ink jet printer is characterized by a high recording speed, a low recording cost and easy realization of color print. As the ink droplet ejection system in this ink jet printer, a thermal system using a heating element as an electrothermal conversion element is known.
The printer of the thermal ink jet system has a recording head which has an ejection port for ejecting flying droplets (hereinafter also referred to as droplets) of ink for recording, an ink flow path communicating with the ejection port, and an electrothermal conversion element provided at a portion of the ink flow path for providing ejection energy for forming droplets. In this ink jet printer, the recording head provides ejection energy to the ink in the ink flow path by applying a drive pulse to the electrothermal conversion element at every arrival of the recording head at a recording position in accordance with the movement thereof, and thus ejects the ink as flying droplets from the ejection port. Then, the ink jet printer impacts the droplets on a recording medium such as paper, thus forming dots thereon. The dots formed on the recording medium constitute a dot matrix in accordance with the movement of the recording head. The ink jet printer records characters and images using the dot matrix.
In the above-described recording device, generally, the recording head has, for example, a plurality of ejection ports in the moving direction (main scanning direction) and in the direction perpendicular to the moving direction (sub scanning direction). The moving direction of the recording head is referred to as “main scanning direction” and the direction perpendicular to the main scanning direction is referred to as “sub scanning direction.” In the ink jet printer, though all the electrothermal conversion elements can be simultaneously driven in recording, it is considered to divide the plurality of electrothermal conversion elements into several blocks and carry out time-division drive for sequentially driving the respective divided blocks in a time-divisional manner, in order to avoid a large burden on a power supply unit for supplying power to the recording head.
Moreover, when recording an image or the like on paper as a recording medium, the ink jet printer generally uses image processing such as a so-called dither method or an error diffusion method to express the gray scale and prints the image by pseudo gray scale expression. Normally, various image quality modes are provided in the ink jet printer, and the ink jet printer records one line in the main scanning direction with one nozzle or records one line with a plurality of nozzles by utilizing the movement of the paper fed in the sub scanning direction. Particularly when printing an image of high quality, the ink jet printer uses the latter method for recording with a plurality of nozzles and shorten the moving distance of the paper in the sub scanning direction, thereby making correction so that unevenness in the dot impact position causing a longitudinal stripe in the paper feed direction, that is, a so-called banding noise, becomes inconspicuous.
The recording head in the ink jet printer may be a so-called serial head with a length smaller than the page width of the paper, or a so-called line head with a length substantially equal to the page width of the paper. The line head is a recording head which enables substantially simultaneous recording in the direction of the width of the paper. Unlike the serial head, the line head does not move in the main scanning direction. That is, the ink jet printer having the line head is characterized in that the line head or the paper moves only in the sub scanning direction and that a very large number of nozzles are provided in the longitudinal direction of the line head. For example, with a pitch of 600 dpi (dots per inch), 5100 nozzles per an 8.5-inch width are provided.
Meanwhile, in carrying out multiple gray scale recording in the ink jet printer, the following two problems arise.
The first problem is that the ink jet printer having the line head cannot adopt the recording method used in the ink jet printer having the above-described serial head. As the recording method in the ink jet printer having the line head, it is considered effective to use a PNM (pulse number modulation) system which impacts ink droplets a plurality of times to overlap, thus forming one dot. However, the use of the PNM system increases the number of ejection pulses per pixel, and also in consideration of the number of nozzles in the line head, (number of nozzles) (pulse number) must be controlled in the ink jet printer. Thus, there arises a problem that the dissipation power tends to be larger than in the case of the serial head.
The second problem is that, in the ink jet printer having the line head, since the line head does not move in the main scanning direction, the respective lines print respective lines. Moreover, the ink jet printer having the line head cannot adopt the recording method used in the ink jet printer having the serial head. Therefore, the image quality is deteriorated by nonuniformity, a streak or the like due to the unevenness in the dot impact position on the paper.
Furthermore, since the ink jet printer having the line head carries out the above-described time-division drive, the ink ejection timing differs among the nozzles. Therefore, there arises a problem that a positional shift of dots occurs in the main scanning direction, causing deterioration in the image quality.